1. Field of the Invention
The present invention relates to a read out apparatus for reading out information from a magneto-optic disk.
2. Description of the Related Art
Magneto-optic disks are becoming popular because of the large memory capacity, high reliability, and the like. Hence, the magneto-optic disk is applicable to various fields including recording and read out of image information and recording coded data for use in computers.
A description will be given of the basic principle of recording and reading out information on and from a magneto-optic disk with reference to FIGS. 1A and 1B.
First, as shown in FIG. 1A, an external magnetic field of a magnet 11 is applied on a magneto-optic disk 10, and an erasing beam 12 is thrown on a part where information is to be rewritten. The part which is irradiated by the erasing beam 12 is heated, and the direction of magnetization at this part is arranged in one direction.
Then, as shown in FIG. 1B, the direction of the external magnetic field of the magnet 11 is reversed. A recording light beam 13 is thrown on the magneto-optic disk 10 in accordance with data Dw shown in FIG. 2(A) which is to be recorded as shown in FIG. 2(B). Hence, the magneto-optic disk 10 is selectively heated and magnetic domains D having opposite directions of magnetization to that of surrounding area are formed so that edge positions of the magnetic domains correspond to the data "1"s of the data Dw to be recorded as shown in FIG. 2(C). The magnetic domains D are formed on tracks or track turns of the magneto-optic disk 10.
There are two methods of forming the magnetic domains D. The mark position recording method forms the magnetic domains D in accordance with the data "1"s of the data Dw to be recorded. On the other hand, the mark edge recording method (or the mark length recording method) forms the magnetic domains D so that leading or trailing edges correspond to the data "1"s of the data Dw to be recorded. According to the mark position recording method, it is difficult to improve the recording density because the recorded data Dw is made to correspond as it is to the direction of the magnetization. However, the mark edge recording method is a compression technique which makes the data "1"s of the data Dw to be recorded correspond to the edges of the magnetic domains D, and it is possible to greatly improve the recording density.
FIGS. 2(A) through (C) respectively show the data Dw to be recorded, the light emitting pattern of the recording light beam 13, and the recorded magnetic domains D on the magneto-optic disk 10 for the case where the edge recording method is employed. Hence, the recording light beam 13 is turned ON and OFF as shown in FIG. 2(B) in accordance with the data "1"s of the data Dw to be recorded shown in FIG. 2(A), so that the edges of the magnetic domains D on the magneto-optic disk 10 correspond to the data "1"s of the data Dw to be recorded as shown in FIG. 2(C).
When reading out the recorded information from the magneto-optic disk 10, a read out light spot Pr scans the magnetic domains D as shown in FIG. 3(A). A read out signal Sr shown in FIG. 3(B) is obtained by the scan of the read out light spot Pr, and read out data Dr shown in FIG. 3(C) can be read out by detecting the edges of the read out signal waveform shown in FIG. 3(B).
Various systems have been proposed for reading out information from the magneto-optic disk, and examples of such systems are disclosed in Japanese Laid-open Patent Application Nos. 61-214278 and 63-53722.
FIG. 4 shows an example of a conventional read out system. The read out system shown in FIG. 4 includes a head 111, an amplifier 112, a signal processing circuit 113, a phase-locked loop (PLL) circuit 114, a data separator 115, and a decoding circuit 116, which are connected as shown. When the read out light spot Pr scans tracks of the magneto-optic disk 10, the read out signal Sr shown in FIG. 3(B) is output from the amplifier 112. The signal processing circuit 113 processes the read out signal Sr and outputs an edge signal Se shown in FIG. 3(D) which indicates the rising and falling edge positions of the read out signal Sr. A clock signal is formed in the PLL circuit 114 based on the edge signal Se, and the data separator 115 obtains the read out data Dr shown in FIG. 3(C) based on the clock signal and the edge signal Se. Since the read out data Dr takes the form of a run length limited code suited for the recording on the magneto-optic disk 10, the read out data Dr (code) is converted into a normal digital data in the decoding circuit 116.
Generally, the edge positions of the read out signal Sr are detected by the use of a threshold value L shown in FIG. 3(B). This threshold value L is a center value between maximum and minimum values of the read out signal Sr, and the intersections of the read out signal Sr and this threshold value L are detected as the edge positions of the magnetic domains D.
FIG. 5 shows a data format on the magneto-optic disk 10. In order to manage the recorded data, each track of the magneto-optic disk 10 is divided into ten-odd sectors. A sector mark Ms which indicates the start of the sector is recorded at the head of each sector, and an identification (ID) number Mi which specifies each sector is recorded after the sector mark Ms. The sector mark Ms and the ID number Mi are physically formed pits of .lambda./4 in depth, where .lambda. denotes the wavelength. Variable frequency oscillator (VFO) pull-in domains are recorded in a VFO pull-in area Mv and phase adjusting domains are recorded in a synchronized byte (SB) area Ms, both by magnetic means, following the ID number Mi. Further, the data is recorded in a data area Md following the SB area Ms. The VFO pull-in domains are made up of magnetic domains which have a predetermined length and are arranged at predetermined intervals.
When the operator specifies the data which is to be read out at the time of the read out, the head 111 moves to the sector which contains the specified data. Then, after confirming that the ID number of this sector matches the ID number of the target sector which contains the specified data, the read out signal Sr shown in FIG. 3(B) is obtained by reading the row of the VFO pull-in domains recorded in the VFO pull-in area Mv, and the edge positions of each of the domains are detected from the edge signal Se shown in FIG. 3(D). As described above, the VFO pull-in domains are made up of magnetic domains which have a predetermined length and are arranged at predetermined intervals. Accordingly, by supplying to the PLL circuit 114 the edge signal Se which is obtained based on the read out signal Sr of the VFO pull-in domains, it is possible to adjust the frequency of the clock signal to a predetermined frequency prior to the data read out. In addition, it is also possible to adjust the phase of the frequency-adjusted clock signal using the domains of the SB area Ms.
However, according to the edge recording method, the edges of the magnetic domains correspond to the data "1"s of the data to be recorded. For this reason, unless the recording is made so that the length of the magnetic domains accurately matches a predetermined length, there is a problem in that the decoded read out data will not match the data to be recorded.
On the other hand, the magneto-optic disk is heated by a laser beam at the time of the recording. Hence, there is a problem in that the length of the magnetic domains becomes different at parts of the magneto-optic disk even if the recording is carried out at the same laser power, due to inconsistent heating conditions, a change in ambient temperature, non-uniform heat sensitivities at various parts of the magneto-optic disk, and the like. Furthermore, there is a problem in that the length of the magnetic domains becomes different among the individual magneto-optic disks due to non-uniform heat sensitivities among the magneto-optic disks and the like.